The present invention relates to power actuated fastener tools.
Power actuated tools for driving a fastener such as a nail, into a substrate, such as a concrete beam, conventionally comprise a barrel from which the fastener is expelled by means of a piston driven by detonation of an explosive charge. The charge is fired by release of a firing pin after cocking of the tool. In conventional firing mechanisms the firing pin has a slot of sufficient depth to allow a pawl to be contained within the slot. A spring inside the slot biases the pawl to project out of the slot to engage with, and be retained by, a cocking piece. To cock the tool, the forward end of the barrel is pressed hard against the work surface and this results in the rear face of the barrel pushing against the cocking piece which retracts the firing pin against the bias of a firing pin spring. When the firing pin is in the cocked position, a sear which is connected to the trigger is aligned with the pawl. When the trigger is actuated, the sear retracts the pawl into the firing pin, thus releasing the firing pin from the cocking piece whereby the firing pin is driven by the firing pin spring towards the explosive charge to fire the power actuated tool.
The firing pin is of relatively small diameter and the space available for mounting the pawl within the firing pin is limited. For the tool to perform consistantly the fit between the pawl and the slot in the firing pin must be maintained. It has been found that repeated firing results in the pawl wearing against both the cocking piece and the slot wall. This results in the pawl becoming loose within the cocking piece and firing pin. Consequently when the sear applies the upward force upon actuation of the trigger, instead of driving the pawl upward into the slot of the firing pin, the pawl may twist and jam. Thus a greater release force is required which makes actuation of the trigger increasingly difficult. It has also been known for a pawl to jam and then to suddenly, and unexpectedly, release, thus firing the tool, which can be very dangerous.